1. Field of the Invention
The invention relates to a plastic cable with at least one electric conductor which is surrounded by a cable covering which includes in addition to at least one insulating layer, several electrically conducting layers of a polymer material.
2. Description of the Prior Art
Plastic cables of the above type are preferably used in electric power systems with rated voltages of 20 to more than 100 kV. The multi-layer conductor covering of these plastic cables, besides the insulation layer proper, also includes conductive layers or tapes. They are provided for smoothing out the contours of the metallic conductors used in the cable and to generate a radially homogeneous electric field in the insulation.
In the presently known plastic cables, the conductive layers are made of filled polyolefins which are extruded concurrently with the insulation in the same operation. The conductivity of these polyolefins is brought about by fillers such as carbon black and graphite. Polymer material with these additives has the disadvantage that with a small content of these fillers, the conductivity is not significantly increased and that with the addition of a certain amount of additives on, the conductivity then increases in step-fashion in such a manner that a defined conductivity of the plastics cannot be set reliably in the range of interest. Also, even with the finest possible grain of the fillers, field-distorting inhomogeneities can occur which lead to a reduction of the dielectric strength of the cable.